A model system consisting of isomerically and radiochemically pure all-trans-(15-14C) and (11,12-3H) retinoic acid (vitamin A acid), H2O2 and horseradish peroxidase was used to generate decarboxylation and oxidation products of retinoic acid in vitro. A major product was purified by high performance liquid chromatography (HPLC). Based on the mass spectral fragmentation pattern, a molecular ion of 302 and peak matching of the NMR spectra by LAOCN3, the major decarboxylation product was identified as a 4-oxo-C19 aldehyde with a hydroxyl group on the side chain at C9, specifically 8-(2,6,5-trimethyl-3-oxo-cyclohex-1-enyl)-2,6-dimethyl-6-hydroxyoctatrienal. Eleven isomers of methyl retinoate were produced by irradiation of methyl all-trans-retinoate in dimethyl sulfoxide with fluorescent light. The isomers were purified by HPLC and the proton NMR spectrum of each isomer was matched by the LAOCN3 program. These isomers have been identified as a 13-cis-(5 yields 10)-photocyclized isomer, the 9, 11, 13-tri-cis-, 11, 13-di-cis-, 7, di-cis-, 13-cis-, 5, 13-di-cis-, 13-trans-(5 yields 10)-photocyclized,-, 11-cis-, 9-cis-, 7-cis-, and all trans-isomers. The photocyclized isomers arise from methyl 7, 13-di-cis-retinoate and methyl 7-cis-retinoate, respectively, via cyclization of the 5, 7 and 9 double bonds.